Typical optical fibers include a core material and a cladding material, with each of the core and the cladding traditionally being fabricated largely from fused silica and having dissimilar refractive indices. A fiber can be formed by drawing a preform, which is a large mass that includes the core and cladding materials arranged in a shape that can resemble the cross-section of the finished fiber, but is much larger. The preform can be drawn in a fiber drawing tower during which the preform is heated and stretched from one end to form the optical fiber.
Optical fibers need to be able to operate at progressively higher power levels as performance demands continue to grow. A leading impairment to high power systems, and particularly high-power-per-unit-bandwidth systems, is stimulated Brillouin scattering (SBS), which is an acousto-optic interaction that manifests noise in low power applications and acts as a highly-efficient power reflector in high power applications. While there are optical fiber-based applications that do not have strict requirements on the laser spectrum, many systems such as spectroscopic lidar systems and coherently-phased fiber laser arrays rely principally on the ability to achieve a desired wavelength and spectral purity. SBS remains a serious stumbling block to the improvement of such systems.
In an attempt to decrease SBS, a variety of silica-based fibers possessing unconventional core compositions have been developed. Exemplifying such silica-based novel fibers are those derived using yttrium aluminum garnet (YAG) crystals. Unfortunately, while these materials show promise in decreasing SBS, such core compositions are very difficult if not impossible to realize using the chemical vapor deposition methods conventionally employed to manufacture optical fiber. Fiber designs including tailored acoustic velocity profiles of the fiber core and cladding that spread the SBS interaction over multiple acoustic frequencies have also been employed to lessen SBS. With careful fiber design, pure silica clad fibers with alumina-doped silica cores have also been formed to increase the acoustic wave loss, thereby decreasing the nonlinear interaction time and the Brillouin gain coefficient. While each of these complex and costly fabrication methods have been proven to partly suppress SBS or offer moderate enhancements in power levels, sufficient narrow-linewidth power scaling will remain impossible without a paradigm shift in the approach to the management of SBS.
What are needed in the art are optical fibers that exhibit extremely low, even zero, Brillouin scattering. In addition, what are needed in the art are facile, economical methods for forming such fibers.